Numerical simulations of synthetic jet based separation control in a canonical separated flow

Direct simulations of a flow configuration devised for investigating active separation control using zero-net mass-flux (or synthetic) jets are presented. The computational configuration consists of a 5% thick flat plate with 8:1 elliptic leading edge and blunt trailing edge at zero incidence in a free-stream. A separation bubble of prescribed size is created on the top surface of the airfoil at the aft-chord location by applying blowing and suction on the top boundary of the computational domain. Such separated flows are generally characterized by three distinct time scales corresponding to the shear layer, the separation zone, and the vortex shedding in the wake; therefore the resulting flow field can be considered as a canonical separated airfoil flow. Two-dimensional simulations of this flow at a chord Reynolds numbers of 60,000 subject to zero-net mass-flux (ZNMF) perturbation of the boundary layer at different characteristic time scales are investigated. Results indicate that at a Reynolds number of 60,000, the entire system comprised of the shear layer, the separation zone, and the wake is locked on to a single frequency. Forcing the ZNMF device close to this lock-on frequency or its first superharmonic is found to result in optimal control of the mean separation bubble. The stability characteristics of the different mean flows resulting from ZNMF forcing at different frequencies are evaluated in terms of local linear stability theory based on the Orr-Sommerfeld equation. The numerical results are also Fourier analyzed in time and compared to the theoretical results.